The invention concerns a solar-powered radio-controlled timepiece wherein an energy storage means provides energy for operating a transducer of a time display.
A radio-controlled timepiece of that kind is known from U.S. Pat. No. 4,714,352 (corresponding to EP-A-0 208 986). That publication describes interrogation of the energy storage means for the photovoltaically generated electrical charges in respect of two different levels so that, with a rising stored voltage, at the commencement of operation, initially only the consumers or loads which are necessary for the start-up procedure are set in operation. The other consumers or loads are only switched on when the storage means has reached a higher energy level as a result of further recharging. That ensures that an overload does not occur immediately at the beginning of a rise in voltage at the storage device, with the consequence of the voltage collapsing, because otherwise operation of the timepiece, which has just begun, would be immediately switched off again, without the attainment of a stable operating state.
The functions which are necessary at the commencement of operation for the attainment of a finally stable operating state for the timepiece include operation of the processor and the receiver of a radio-controlled timepiece of the general kind set forth, as is described in greater detail for example in U.S. Pat. No. 4,645,357 (corresponding to EP-B-0 180 880). Detection of the status of the hands of the radio-controlled timepiece results in automatic correction of the positions of the hands as soon as it was possible for an applicable current item of time information to be received by radio and decoded in the processor.
The user therefore does not need to perform any manipulation operations for commencing operation; as soon as there is sufficient light energy for an adequate state of charge of the storage means, the timepiece automatically starts operating and finally automatically sets itself to the correct time display.
A problem that is involved however is that towards the end of operation an initially unnoticeable and then slowly increasing incorrect hands display can occur. That is not acceptable for a radio-controlled timepiece which is advertised as the most accurate of all conceivable consumer timepieces and which enjoys a high level of market acceptance precisely because of that accuracy--as well as because of the absence of operating requirements.
Therefore the object of the present invention is to avoid such incorrect displays which occur if towards the end of operation the energy still contained in the storage means is admittedly still sufficient for the separately buffered operation of a processor for the electro-optical display control, but not for reliable operation of the electromechanical transducer for a time-keeping movement of the hands. More specifically the latter is no longer guaranteed if, with the storage means voltage having fallen to a critical level, the drive pulse for the motor movement admittedly starts, but then does not persist over its fully predetermined length but, as a result of the electrical loading of the storage means which is already almost discharged, results in temporary breakdown of the residual voltage.
The consequence of this is that neither of the at least two shorter partial pulses which occur as a result supplies sufficient energy for operation of the motor. However, the other functions which are directly initiated by the processor and which in particular are electro-optical, such as the continuing digital representation of the seconds in a display, are finally continued without disturbance so that, towards the end of operation, the actual hands position increasingly lags behind in comparison with time-derived information which is displayed in other ways, and naturally also in relation to the actual elapse of time.
No less operationally critical is the fast-motion drive phase for rapidly pivoting the hands into a given angular position (see U.S. Pat. No. 4,650,344, corresponding to EP-B-0 180 155), in particular in the course of the hands position check on the basis of the time which is communicated by radio, the checking operation being automatically initiated at the commencement of operation and then from time to time for a hands position correction movement which may be necessary. For, the high stepping speed which occurs in the correction phase of movement, with a correspondingly increased dynamic loading on the stepping motor due to the moment of inertia of the movement transmission, represents an extreme level of loading in regard to the energy source and the drive system, and for that reason now the risk of the loss of a motor step is particularly great. If a motor step is not also actually implemented with each stepping pulse (and even in the event of interrupted motor pulses), the result of the motor drive pulse counting procedure is no longer in conformity with the number of steps actually effected so that the hands are no longer rotated into the angular position that is predetermined by the required number of steps, and therefore give an incorrect display.